data migration for MailMessages

[Dries Desmet]

Since the website doesn't work if there is no data in MailMessages, we need to write a datamigration. This will smooth out the upgrade on the production server.

[Ruben Van Wassenhove]

resolve #20

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[Dries Desmet]

Merge commit '5a9d9de' into develop

• commit '5a9d9de': (21 commits) resolve #23 resolve #19 test & fix #17 fix merge_usergroups if there is no false data i tried and i failed resolve #21 resolve #22 resolve #18 custom admin index template with campaignmonitor login form update gitignore resolve #20 working data migration for mailmessages before data migration is subscribed with icon execute dump_cm_subscribers when loading changelist_view working management command create dump_cm_subscribers.py subscribe & unsubscribe in model i.s.o. admin minor changes BadRequest fallback ...

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